This invention relates to a method and an apparatus for determining the position of a weld groove relative to a predetermined desired position for the purpose of correcting the relative position of the laser work beam to the weld groove. The method includes the steps of alternatingly deflecting a collimated laser measuring beam back and forth about the path of the weld groove and focusing the beam, and after reflection from the workpiece in which the weld groove is formed, the beam is received by an electro-optical sensor which is operatively connected to a follow-up device for correcting the position of the laser working beam.
When using a laser for welding, in case of a straight welded seam, preferably the beginning and the end point of a weld groove are determined with the aid of a measuring beam accurately oriented in the direction of the axis of the main or welding laser beam. The determined points are inputted in the control program for the main laser beam and the weld is made according to a predetermined program--in the present case a straight line--between the determined points.
During welding, however, the problem is encountered that a prepared weld seam does not always correspond to the desired course. This may occur, for example, due to improper preparation of the weld groove and/or an imprecise programming of, for example, a non-linear path for the groove. In such a case, the program-controlled main laser beam may be guided out of alignment with the center of the weld groove which leads to grave welding errors.
As described, for example, in "Technisches Messen" (Industrial Measurements) 51, No. 7/8, pages 259-263, seam-seeking systems are known for automatic electric arc welding in which the prepared weld groove is scanned by means of a focused laser measuring beam which sweeps transversely back and forth over the weld groove at a measuring frequency of maximum 10 Hz. If the weld groove deviates from the desired course, the position of the laser welding beam or the welding torch relative to the weld groove is corrected accordingly. In these prior art seam-seeking systems, a row of photodiodes are employed as sensors. Due to the large number of measuring locations and the time required for evaluation, these seam-seeking systems operate at an accuracy of only about 0.2 to 0.5 mm.
Further, from the art of measuring surface profiles and roughnesses and from the optical memory art (CD [compact disc] art) measuring systems are known which operate with a very high measuring accuracy of about 1 .mu.m. These systems, however, must be brought very close (generally no more than 1 mm) to the object being measured.
The diameter of the focal spot of a focused laser working or welding beam is sometimes only 0.1 to 0.2 mm. Due to the very small size of the laser spot, the weld grooves provided for laser welding also have only very small cross-sectional dimensions. The welding of such weld grooves requires a measuring accuracy in an order of magnitude of half the diameter of the laser spot, that is, about 0.05 mm. The measuring accuracy attainable with prior art seam-seeking systems employed for electric arc welding is thus not sufficient for this purpose.
The feed rate during laser welding may extend to an order of magnitude of 100 mm/s and is thus generally up to ten times greater than the speed used in electric arc welding. For this reason, the measuring frequency of the measuring systems known for electric arc welding is also insufficient since the measurement signals resulting from the oscillating measuring beam do not occur in a sufficiently rapid sequence to cause the main laser beam to follow accurately at the given feed rates.